JNS 3 (2013) 43 ‐ 51
Sol-Gel Synthesis, Characterization and Optical Properties of Bi3+Doped CdO Sub-Micron Size Materials
Abdolali Alemia, Shahin Khademiniaa*, Sang Woo Joob, Mahboubeh Dolatyari c, Hossein Moradi d
a
Department of Inorganic Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran
School of Mechanical Engineerng WCU nano research center,Yeungnam University,Gyongsan 712-749 , South
KOREA
c
Laboratory of Nano Photonics & Nano Crystals, School of Engineering-Emerging Technologies, University of
Tabriz, Tabriz, Iran
d
Faculty of chemistry, Islamic Azad University, Ardabil Branch, Ardabil, Iran
b
Article history:
Received 11/1/2013
Accepted 4/5/2013
Published online 1/6/2013
Keywords:
Sol-Gel Method
Cadmium Oxide
Bismuth
Optical Properties
PXRD
*Corresponding author:
E-mail address:
[email protected]
Phone: +98 9116224110
Fax: +98 1326373157
Abstract
Highly crystalline Bi3+-doped cadmium oxide (CdO) sub-micron
structures were synthesized by calcination the obtained precursor
from a sol-gel reaction. The reaction was carried out with cadmium
nitrate (Cd(NO3)2.4H2O), bismuth nitrate (Bi(NO3)3.5H2O) and
ethylene glycol (C2H6O2) reactants without any additives at 80°C for
2h. Resulting gel was calcined at 900 °C with increasing temperature
rate of 15°C per minute for 12 h in a furnace. As a result of heating,
the organic section of gel was removed and Bi3+-doped cadmium
oxide micro structure was produced. The obtained compound from
the sol-gel technique possesses a cubic crystalline structure at micro
scale. Powder x-ray diffraction (PXRD) study indicated that the
obtained Bi3+-doped CdO has a cubic phase. Also, SEM images
showed that the resulting material is composed of particles with the
average diameter of 1 µm. Also, UV–vis and FT-IR spectroscopies
were employed to characterize the Bi3+-doped CdO micro structures.
2013 JNS All rights reserved
1. Introduction
Over the last decades, Bi2O3-based materials with
high oxygen ionic conductivity have been extensively
studied for their potential use as solid electrolyte in
fuel cell [1], high purity oxygen generators and
electrochemical sensors [2]. Also bismuth oxide is an
important metal-oxide semiconductor [3]. Owing to
these peculiar characteristics, Bi2O3 has been studied
in various fields and is widely used in electrolyte,
electro reduction, and sensor optical coatings as well
as in transparent and superconductor ceramic glass
44
manufacturing [4]. For these reasons we chose Bi2O3
as dopant material. Also, the films of transparent
conductive oxides (TCO) such as zinc oxide and
cadmium oxide (CdO) have been extensively studied
because of their use in semiconductor optoelectronic
device technology [5]. CdO films have been
successfully used for many applications, including use
in gas sensor devices, photo diodes, transparent
electrodes, photo transistors, and photovoltaic solar
cells [6]. Also, CdO is an n-type semiconductor with a
cubic crystal structure, possesses a direct band gap of
2.2 eV [7]. Beside, CdO shows very high electrical
conductivity even without doping due to the existence
of shallow donors caused by intrinsic interstitial
cadmium atoms and oxygen vacancies [8]. In previous
works, synthesis of Sn-doped CdO thin films [9], Bi3+doped CdO thin films by sol–gel spin coating method
with different raw materials and heat treatment that
resulted different morphology[10], copper doped CdO
nanostructures [11], ZnO doped CdO materials [12],
titanium-doped CdO thin films [14], ZnO–CdO–TeO2
system doped with the Tb3+and Yb3+ ions [15], Ndoped CdO [16], samarium, cerium, europium, iron
and lithium-doped CdO nanocrystalline materials [13,
17, 18, 19, 21], indium doped CdO films [20], gallium
doped CdO thin films [22], Gd doped CdO thin films
[23], Li–Ni co-doped CdO thin films [24], aluminumdoped CdO [25, 26], fluorine-doped CdO Films [27],
La doped CdO[28], Dy doped CdO films[29], carbon
doped CdO[30], Mn Doped Nanostructured CdO[31],
boron dope CdO [32] have been performed.
In this work, crystalline Bi3+ doped CdO sub-micron
size structures have been synthesized by sol-gel
method, with cadmium nitrate, bismuth nitrate
(Bi(NO3)3.5H2O) and ethylene glycol (C2H6O2) as raw
materials without using any catalyst or template at a
heat treatment temperature of 900°C with increasing
temperature rate of 15°C per minute for 12h reaction
time, which is a very simple and economical method.
A.Alemi et al./ JNS 3 (2013) 43‐51
Also, we discussed about dopant concentration effect
on the morphology of the synthesized materials. The
product was characterized by PXRD, SEM, FT-IR and
UV-vis techniques.
2. Materials and Methods
All chemicals were of analytical grade, obtained
from commercial sources and used without further
purification. Phase identifications were performed on
a powder X-Ray diffractometer Siemens D5000 using
Cu Kα radiation (λ=1.542Å). The morphology of the
obtained materials was examined with a Philips XL30
Scanning Electron Microscope (SEM). Absorption
spectra were recorded on a Jena Analytik Specord 40.
Also FT-IR spectra were recorded on a Tensor 27
Bruker made in Germany.
2.1 Synthesis of BixCd1-xO micro-size layer (x =
2, 3 and 3.2%)
4.78 mmolar (1.958 mmol), 4.73 mmolar (1.94
mmol) and 4.72 mmolar (1.935 mmol) (Mw=308.482
g.mole-1) cadmium nitrate (Cd(NO3)2.4H2O), 0.098
mmolar (0.04 mmol), 0.146 mmolar (0.06 mmol) and
0.156 mmolar (0.064 mmol) (Mw=485.071 g.mole-1)
bismuth nitrate (Bi(NO3)3.5H2O), respectively and
10 ml ethylene glycol (C2H6O2) were added in 400 ml
distilled water. Then, the solution was stirred at 80 °C
for 2h until a dried gel was obtained. The gel was
brown color and spongy. The dried obtained gel was
treated thermally at 900 °C for 12 h. After the reaction
completed, and cooled slowly to room temperature,
the obtained material was pulverized. The sample was
black like powder.
A.Alemi et al./ JNS 3 (2013) 43‐51
3. Results and discussion
3.1 PXRD analysis
In order to investigate the structural properties of
Bi3+ - doped CdO micro structures X-ray diffraction
measurement varying the diffraction angle, 2, from 4◦
to 70◦ were performed. Powder X-ray diffraction
(PXRD) patterns of CdO micro structures calcinated
at 900 ◦C in air are shown in Fig. 1. The diffraction
peaks at 2θ values of 33.10°, 38.32°, 55.32°, 65.96°
and 69.31° matching with the 111, 200, 220, 311, and
222 of cubic CdO (JCPDS-05-0640) indicated the
formation of CdO with excellent crystallinity. Fig. 1
represents the XRD patterns of the obtained materials
after 12 h thermally reaction time at 900°C at xBi=2, 3
and 3.2 mmole, respectively. Fig. 1(d) shows that with
increasing the dopant amount to xBi3+=3.2 mmol, some
peaks arises in about 2Ѳ≈37, 56° that assigned to
Bi2O3 [33-38]. So the doping limitation is x=0-3
mmole. The inter planar spacing (d) was calculated
45
follow: 4.6912 Å while for pure CdO, the cell
parameter is 4.6950. So the pure CdO and Bi doped
CdO cell volume are 103.4920 and 103.2409,
respectively. Also, Crystal sizes of the obtained
materials were measured via Debye Scherer’s
equation t = 0.9λ , where t is entire thickness of
B1 cos θB
2
the crystalline sample, B1/2 of FWHM is the full
width at half its maximum intensity and ѲB is the
half diffraction angle at which the peak location
is, that are as follows: 28.2, 26.3, 25.4, 23.4 nm for
pure CdO, and 2, 3 and 3.2 dopant concentration
mmole. The pattern shows polycrystalline structure of
cubic CdO structure (NaCl structure of a space group
Fm3m). The lattice constant calculated for an undoped
CdO sample, was a=0.46950 nm (JCPDS 05-0640).
The PXRD measurements confirm that a pure phase
of the cubic CdO is formed [39-45].
via Bragg’s law (nλ = 2dhkl sin θ)) where n is called
the order of reflection (we used n=1), d is
interplanar spacing, Ѳ is the half of diffraction
angle and λ is the incident X-rays of wavelength.
Because the radii of Bi3+ (r=0.96Å [1]) is smaller than
the radii of Cd2+ (r=1.55 Å), compared to PXRD
patterns of the pure CdO, the diffraction lines in the
PXRD patterns of Bi3+- doped CdO shift to higher 2Ѳ.
So (∆2θ=33.10 (doped) - 32.97 (pure) =0.13),
(∆d=2.7135 (pure) - 2.703 (doped)=0.0105Å).
Because Bi2O3 (fcc crystal phase) and CdO have cubic
crystal structure, and so they are iso-crystal phase,
when Bi is coming into CdO unit cell in the place of
Cd, because the radius of Bi is smaller than Cd, so
there is a contraction in the CdO unit cell. So we
conclude that as a result of the contraction in the
obtained crystal the inter planar space will be reduced.
Also, with using celref software version 3, the cell
parameter for highest amount of Bi doped CdO is as
Fig. 1. PXRD patterns of the synthesized Cd1-xBixO micro
size materials, where (a) is pure CdO, (b) x = 2; (c) is x = 3;
and (d) is x = 3.2 mole%.
A.Alemi et al./ JNS 3 (2
2013) 43‐51
4
46
3 Microstrructure ana
3.2
alysis
Fig. 2 reveaals the SEM images
i
of cubbic structure of
o
obtained
crysttalline CdO at
a 900 °C [45]. Remarkablly,
it was observeed that the av
verage particlle size is at thhe
r
range
of 2 µm
m. As shown in
i fig. 2(a) annd (b), it’s cleear
that the matterial is com
mposed of particles wiith
h
heterogeneous
s size. Fig. 2((c) shows thaat the sample is
c
composed
of multigonal particles
p
with heterogeneouus
s
size.
Fig. 2(dd) and (e) show that the morphology is
c
clearly
layereed like, as a resulted of calcinatioon
treatment, witth the particlee almost spherrical shape annd
the width sizee in a range off about 1 to 3 µm.
T SEM imaages of the sy
The
ynthesized Bi3+-doped CddO
m
micro-size
maaterials are giiven in Fig. 3 and 4. Fig. 3
s
shows
SEM images off Cd0.98Bi0.022O micro-sizze
p
particle.
The average wiidth size off the sphericcal
s
shaped
particles that form
med layer struuctures is in a
r
range
between 1 to 2 µm
m. Since the images
i
for thhe
p
pure
and dopeed materials are
a not similarr we concludeed
the dopant cooncentration at which thee material was
o
obtained
has influence on
n the particlee morphologgy.
F 3(a) and (b) show th
Fig.
hat there are cavities in thhe
f
form
of maccro-porous th
hat caused from
fr
thermallly
treatm
ment at calcinnations in 9000°C. Fig. 3(cc) and (d)
show that due to doping
d
Bi3+, there
t
are morre cavities
o
mateerial. In this figure
f
it is
as porrosity in the obtained
clear that
t
the partiicle sizes aree heterogeneoous. Also,
it’s clear that theree are particless on the surfaace of the
s
of them is about 5000 nm to 1
materiial that the size
µm. Fig.
F 3(e) and (f) show highh magnificatiion of the
cross section of eaach particle and
a it seems that they
c
layerrs. Fig. 4 shoows SEM
are coomposed of compact
imagees of Cd0.97Bii0.03O micro-ssize particle. Fig. 4(a)
and (bb) show botth macro-porrous structuree and the
cavityy in the parrticle compoosed the layyered like
materiial. In Fig. 4(c)
4
and (d) it
i seems that there are
small particles ass uncus on the surface of each
particlle. With highher magnificaation in fig. 4(d), it is
clear that
t
the cavitty size is aboout 500 nm. With low
magniification in fig.
f
4e and (f),
( it seems that with
increaasing the doopant amounnt to 3 moole%, the
morphhology of thee particles are changed too particles
with regular shappe and edge,, but the sizze of the
a the macroo-porosity
particlles is still hetterogeneous and
in the structure is clear.
c
Fig.. 2. The SEM images
i
of the synthesized
s
purre CdO micro size materials.
A.Alemi et al./ JNS 3 (2013) 43‐51
Fig. 3. The SEM images of the synthesized Cd0.98Bi0.02O micro size materials.
47
A.Alemi et al./ JNS 3 (2013) 43‐51
48
Fig. 4. The SEM images of the synthesized Cd0.97Bi0.03O micro size materials.
3.3 Spectroscopic studies
Fig. 5 shows the electronic absorption spectra of the
synthesized Bi3+ -doped CdO micro-size materials.
According to the spectra, Bi2O3 presents the
photoabsorption properties from UV light region to
visible light, shorter than 470 nm, which is assigned to
the intrinsic band gap absorption. So in fig. 4a in xBi3+
= 2 mole%, there is a broad band in about 370 nm that
is behind the CdO band and a weak peak in around
1050 nm that corresponded to CdO [39, 46, 47]. With
increasing the dopant amount to 3 mmole, there is a
A.Alemi et al./ JNS 3 (2013) 43‐51
broad band in a range of about 420 nm (Egap = 2.95
eV), partially behind the CdO band that is
corresponded to Bi3+ [3, 34, 35].
Fig. 5. The electronic absorption spectra of the synthesized
Cd1-xBixO micro size materials, where (a) is x = 2 and (b) is
x = 3 mole%.
Fig. 6 shows FT-IR spectrum diagram of the doped
samples. From the present spectrum, it is clear that the
peak at 3443 cm-1 is corresponded to stretching
vibration of H2O molecule [48]. Peak at 1457 cm-1 is
corresponded to carbonate and peaks at 470 and 543
cm-1 are corresponded to CdO [48]. Also we know that
the peaks at 800 to 1400 cm-1 are assigned to CdO
[49]. Peak at 1560 cm-1 can be assigned to a residual
organic component [45]. The intensive signal around
850 cm-1 appeared in FTIR spectra of both 6(a) and
(b) are the stretching vibration of Bi–O bonds in BiO6
octahedra which suggests the stability of
nanocrystalline Bi2O3 during the photocatalytic
reaction [34]. Additionally, several new peaks from
400 to 600 cm−1 attributed to vibration of Bi–O bonds
of BiO6 octahedra [36]. The additional peak at 595
cm−1 is reported to be the vibration of Bi–O bonds of
BiO6 coordination polyhedra in α-Bi2O3 [36]. Beside,
an absorption band at about 1060 cm−1 is also
observed that may be attributed to the other kinds of
49
vibrations of Bi–O caused by the interaction between
the Bi–O bonds and their other surroundings [36].
Fig. 6. FT-IR spectrum of the synthesized Cd1-xBixO
micro-size materials, where (a) is x = 2 and (b) is x = 3
mole%.
4. Conclusion
In summary, micro-layers of Bi3+-doped CdO were
synthesized successfully by employing a simple solgel method. We found that the dopant concentration
affects the morphology of the final product. As shown
by SEM images, with increasing the dopant
concentration, the morphology of the layered like
material was changed. We found that compared to
those of the micro-size material of pure CdO, the
diffraction lines in the powder XRD patterns of Bi3+doped CdO shift to higher 2θ values. The shift in the
diffraction lines might be attributed to the smaller
radius of the dopant ion, compared to the ionic radius
of the Cd2+, which may cause a contraction of the unit
cell and so decrease lattice parameters in the Bi3+doped CdO materials. The synthesized materials
exhibited electronic absorption optical properties in
the UV-visible region, which show dependence on the
dopant amounts in the structure. These materials are
expected to have a potential application in
semiconductor devices, as catalysts, etc.
50
Aknowledgement
The authors expresses their sincere thank to the
authorities of University of Tabriz for financing the
project.
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